In this century, the shortage of fresh water is expected to surpass the shortage of energy as a global concern for humanity, and these two challenges are inexorably linked. Fresh water is one of the most fundamental needs of humans and other organisms. Each human needs to consume a minimum of about two liters per day, in addition to greater fresh-water demands from farming as well as from industrial processes. Meanwhile, techniques for transporting fresh water or for producing fresh water via purification and desalination of seawater, brackish water, waste water, contaminated water, etc. tend to be highly demanding of increasing scarce supplies of affordable energy.
The hazards posed by insufficient water supplies are particularly acute. A shortage of fresh water may lead to famine, disease, death, forced mass migration, cross-region conflict/war (from Darfur to the American southwest), and collapsed ecosystems. In spite of the criticality of the need for fresh water and the profound consequences of shortages, supplies of fresh water are particularly constrained. 97.5% of the water on Earth is salty, and about 70% of the remainder is locked up as ice (mostly in ice caps and glaciers), leaving only 0.75% of all water on Earth as available fresh water.
Moreover, that 0.75% of available fresh water is not evenly distributed. For example, heavily populated developing countries, such as India and China, have many regions that are subject to scarce supplies. Further still, the supply of fresh water is often seasonally inconsistent. Typically confined to regional drainage basins, water is heavy and its transport is expensive and energy-intensive.
Meanwhile, demands for fresh water are tightening across the globe. Reservoirs are drying up; aquifers are falling; rivers are drying; and glaciers and ice caps are retracting. Rising populations increase demand, as do shifts in farming and increased industrialization. Climate change poses even more threats in many regions. Consequently, the number of people facing water shortages is increasing.
Even when fresh water is available, billions of people live with unacceptable levels of contamination. There is a growing need for water purification systems that can remove not only common ions, but also various dangerous trace impurities such as arsenic, copper, radioactive particles, fertilizers, bacteria, viruses, etc. many of which are difficult to eliminate efficiently with traditional filters and membranes. Cheap, low-power, portable systems could have a major impact on public health, if they could be easily deployed to remote or under-developed locations with poor or nonexistent water distribution infrastructure.
Massive amounts of energy are typically needed to produce fresh water from seawater (or to a lesser degree, from brackish water), especially for remote locations. Reverse osmosis (RO) is currently the leading desalination technology, but it is energy intensive and still relatively inefficient due to the large pressures required to drive water through semi-permeable membranes and their tendency for fouling. In large-scale plants, the energy/volume required can be as low as 4 kWh/m3 at 30% recovery, compared to the theoretical minimum around 1 kWh/m3, although smaller-scale RO systems (e.g., aboard ships) can have much worse efficiency, by an order of magnitude.
Rather than extracting pure water, electrochemical methods, such as electrodialysis (ED) and capacitive desalination (CD), extract just enough salt to achieve potable water (<10 mM). Current large-scale electrochemical desalination systems are less efficient than RO plants at desalinating seawater (e.g., 7 kWh/m3 is the state of the art in ED), but become more efficient for brackish water (e.g., CD can achieve 0.6 kWh/m3). These electrochemical methods also offer advantages for efficient high-recovery purification of partially or completely desalinated water, by expending energy mainly to remove just the undesirable particles, rather than most of the water molecules, from the solution. Existing ED and CD methods, however, do not reach the same level of water purity as RO, since some undesirable particles can flow past the electrodes or membranes.